Clearing islands as refugia for black-fronted tern (Chlidonias albostriatus) breeding colonies in braided rivers

Black-fronted terns/tarapirohe (Chlidonias albostriatus) are highly adapted to nesting on clear shingle areas of the braided rivers in the South Island, New Zealand. They are nationally and internationally classified as endangered. Ongoing threats, primarily an interaction of predation and habitat degradation or loss, have resulted in population decline. Conservation management in the form of control of introduced mammalian predators has proven partially successful. Using the lower Waitaki River as a case study, we cleared vegetation from seven islands creating potential refugia from mammalian predators and providing high quality bare gravel breeding habitat. We: (1) determined the mammalian predators present on river banks, vegetated islands and cleared islands; (2) assessed the nesting success of black-fronted terns and primary causes of nest failure; (3) identified the predator species at nests using remote cameras; and (4) compared the nesting success on cleared and vegetated islands. Fewer mammalian predators were detected on islands compared to adjacent riverbanks: mustelids (Mustela spp.) occurred on approximately half of the vegetated islands, but only mice (Mus musculus) were detected once on one of the cleared islands. Black-fronted terns established three colonies on islands immediately after the clearing of vegetation, but nesting success in the lower Waitaki River was low overall (50.5% to 56.4% of nests contained at least one egg that hatched) and the primary cause of nest failure was predation before and after clearing islands. The main predators of nests (62.5% of predation events) were southern black-backed gulls (Larus dominicanus). There was no overall difference in nesting successes of colonies between cleared and vegetated islands, presumably because gulls depredated tern eggs irrespective of vegetation cover around target nests. Nesting success depended on the timing and size of the colony, with earlier established nests and nests in larger colonies being more successful. Artificially created nesting habitat can play a critical role for conservation, particularly on lowland rivers in New Zealand, and we recommend control of avian predators be considered.